EAS 105-THE PLANETS
Prof. Robert L.
Nowack
Lecture 18
Rings and Things
Galileo
first saw the Rings of Saturn in 1610.
He thought they might be bumps or a triple planet. Huygens was first to note there is a flat
ring around Saturn. In 1675, Cassini
found there were 2 rings with a gap in between, now called the Cassini
Division.
Saturn
Since,
by Kepler's Laws, the inner parts of the rings must be orbiting the planet
faster than the outer parts, the rings must not be solid. In fact, the rings are not solid, but made up
of billions of small pieces of debris.
For Saturn, the particles are primarily water ice with sizes from grains
of sand up to house-size
boulders. The rings make up a thin sheet
of particles only 20-30 meters thick all orbiting Saturn. However, the lateral dimension of the rings
is huge. Saturn's radius is about 60,000
kilometers and the rings start from 75,510 km and extend out to 140,180 km or
more.
Each particle making up the rings
follows an almost circular orbit in the equatorial plane of Saturn. (If they didn't follow circular orbits,
collisions would result.)
There
are three distinct rings seen from Earth called A, B, and C. Also,
a narrow F-ring was found in 1979 by the Voyager spacecraft. A thin D-ring was found by Voyager to extend
from the atmosphere of Saturn to the C-ring.
The C-ring is very thin, and Saturn can be seen through it.
At a distance 32,000 kilometers
from Saturn, the brightest B-ring starts. There are few empty gaps and the
particles are so densely packed as to be opaque. One of the stranger features observed for the
B-ring are dark radial spokes that lasts for as long as a rotation and rotates
like spokes on a wheel. These spoke-like
features consist of fine particles that form above the plane of the rings.
Spokes in Saturn’s B-ring
However,
Kepler's Laws should force these spokes to shear and lose their shape. These transient features consist of very fine
particles hovering above the rings. One
possible mechanism involves electrically charged fine particles that rise out
of the ring plane.
At
the outer edge of the B-ring is the Cassini Division. However, this division is not completely
empty. It appears to be some type of
resonance with the nearest medium sized Moon, Mimas, which is much farther out,
but has double the orbital period.
The
A-ring ends abruptly. This also appears
to be some type of resonance feature with nearest medium sized satellite,
Mimas.
Saturn’s A-ring
Beyond
the A-ring is the eccentric F-ring which has a braided appearance. For this case, there appears to be two small
satellites, Prometheus and Pandora, one on each side of the ring. These are called Shepherd Satellites because
of their role in keeping the F-ring narrowly confined.
Shepherd Satellites Prometheus and Pandora
Many of the divisions in the
rings of Saturn are related to gravitational resonances with outer
satellites. There is also significant
fine structure in Saturn's rings.
One is a spiral pattern of light
and dark lines caused by gravitational interaction between the particles called
Spiral Density Waves. Another is Bending
Waves in which there are wrinkles in the rings.
Spiral Density Waves in A-ring
Rings of Uranus
The
rings of Uranus were discovered in 1977 using a method called occultation.
The rings of Uranus as revealed by
measurements of occultation carried out from the Earth. Each dot corresponds to an occultation event
observed at the observatory indicated.
About every 10 years or so,
Uranus passes in front of a bright star blocking it out. This can be used to study the atmosphere of
Uranus just as the star is blocked out by the planet. However, for Uranus, the starlight started to
"wink out” several times before it was “occulted” by the planet. This was inferred by James Elliot and his
colleagues from MIT to be the result of narrow dark rings of Uranus.
A
better look at the rings of Uranus was found from Voyager 2. There appears to be 5 main rings: Alpha, Beta, Gamma, Delta and Epsilon.
These are in some sense opposite
to those of Saturn. Uranus’ rings are
very narrow and dark in contrast to Saturn’s rings which are wide and bright.
Dark Rings of Uranus
The schematic projection below
represents the Uranus ring system on the equatorial plane. Of nine rings, only Epsilon shows appreciable
width.
At least the Epsilon, E, ring
seems to be associated with two shepherding satellites which keep the ring
particles in place by resonant gravitational forces.
Shepherd Satellites Cordelia and Ophelia
Cordelia (1986U7) and Ophelia (1986U8), a
pair of shepherding satellites on each side of Epsilon ring of Uranus keeping
the ring particles in place through resonant gravitational forces. No other shepherds for other rings were found.
Voyager 2 also found several
rings for
Block out of photograph so as to see rings.
A composite portrait of
Finally,
in 1979, even Jupiter was found to have a primary ring, although much more
diffuse than other ring systems.
What
causes rings? Two basic theories have been proposed:
(1) The breakup or collision theory - Rings are the remains of a
shattered satellite or from collision of a comet with a moon.
(2) Rings are original material that never were able to come together
and form a satellite.
In either case, tidal forces play
an important role. These are the forces that tend to
distort a satellite of a larger planet raising bulges both toward and away from
the planet. Around each planet is a
tidal stability limit called the Roche Limit.
The ring systems of Jupiter, Saturn,
Uranus and Neptune compared with the location of the tidal stability limit for
each planet. All four ring drawings are
scaled to the diameters of their respective planet.
Within this distance, small
particles won't come together under their own gravity to form a larger
body. Most ring systems can maintain
themselves only within this Roche Limit.
Of course bodies inside the Roche Limit of a given planet won't get
pulled apart since they typically have some internal strength. Nonetheless, if a satellite near the Roche
Limit were shattered by an impact, the debris could be spread out in this
region. Saturn’s rings contain about as
much material as a small icy satellite of a radius of about 125 kilometers.